1. Field of the Invention
The present invention relates to a method for producing a toner used in recording methods using electrophotography, electrostatic recording and toner jet recording.
2. Description of the Related Art
Energy savings have recently come to be considered as an important issue confronting electrophotographic devices, and significant reductions in the amount of heat required by fixing apparatuses are being examined. Thus, there is a growing demand for toner to be able to be fixed with less energy, or in other words, demonstrate “low-temperature fixability”. One example of a technique for enabling fixation at low temperatures involves lowering the glass transition temperature (Tg) of the binder resin present in the toner. However, since lowering Tg leads to a loss of toner heat-resistant storability, it is considered to be difficult for this technique to simultaneously realize both toner low-temperature fixability and heat-resistant storability.
Attention has particularly been focused in recent years on crystalline resins for use as binder resin materials for enabling simultaneous realization of both low-temperature fixability and heat-resistant storability. Crystalline resins are able to form a structure in which molecular chains composing the resin are arranged in an orderly manner, and are known to have a melting point (Tm) without demonstrating a well-defined Tg. Consequently, they are resistant to softening at temperatures below the crystal melting point, and have the property of demonstrating a sudden decrease in viscosity when the crystals melt after reaching the melting point (sharp melt property).
On the basis thereof, studies have been extensively conducted on toners obtained by adding a crystalline resin to an amorphous binder resin.
However, since crystalline resins are composed of polymeric substances having a certain degree of molecular weight distribution in the same manner as typical resin materials, they are not always able to form a completely ordered structure. Moreover, in the case of using such resins as toner materials, since steps are required in an ordinary toner production process in which thermal hysteresis is applied at a temperature equal to or higher than the melting point or the toner is dissolved in an organic solvent together with other materials, crystalline resins become compatible with amorphous resins resulting in increased susceptibility to the loss of crystallinity. Thus, it is not easy to allow a crystalline resin to be present in a toner while retaining the crystallinity thereof, and normally easily adopts a form consisting of a mixture of portions of high crystallinity and portions of low crystallinity. As a result, there are many cases in which crystalline resins do not demonstrate their inherent “sharp melt property” even if added to toner, while also demonstrating a decrease in heat-resistant storability.
In addition, toners containing components of low crystallinity and low molecular weight components formed as a result of compatibility end up undergoing further decreases in crystallinity due to the effects of these components in the case of allowing to stand for long periods of time. As a result thereof, changes occur in the thermal properties of the toner, thereby causing further decreases in low-temperature fixability and heat-resistant storability.
In order to solve such problems, toner intermediates or toner containing crystalline resin that has decreased in crystallinity is subjected to heat treatment at a temperature lower than the melting point of the crystalline resin in an attempt to reassemble crystalline structure. As a result of polymer chains present in crystalline resins being retained at high temperatures, molecular mobility increases making it easier to form a crystalline structure having a more stable structure. However, in the case of holding at a temperature higher than the melting point, energy ends up being applied that is equal to or greater than the intermolecular force required to maintain crystalline structure, and the crystalline structure ends up collapsing.
For example, Japanese Patent Application Laid-open No. 2006-065015 proposes a toner production method comprising a step in which an intermediate or finished product of a toner production step containing a crystalline polyester and amorphous polyester as binder resins is stored at a temperature of 45° C. to 65° C. According to this method, a compatible portion at the domain interface between the crystalline polyester and amorphous polyester is crystallized. However, in addition to crystallization requiring a long period of time, due to the large difference between the melting point and storage temperature of the crystalline polyester, the effect thereof is not necessarily adequate.
In addition, Japanese Patent Application Laid-open No. 2005-308995 proposes a toner production method comprising melting and kneading a raw material containing a crystalline polyester and an amorphous polyester, followed by carrying out heat treatment, under conditions of a temperature equal to or higher than the glass transition temperature of the molten mixture and 10° C. lower than the softening point of the amorphous polyester, and pulverizing. However, the object of this method is to raise the Tg of the amorphous polyester, and was insufficient for obtaining crystalline polyester having a sharp melt property.
Japanese Patent Application Laid-open No. 2012-042939 proposes a toner production method that involves heating and holding a toner composition containing a crystalline polyester at a temperature (T1) that is 20° C. to 5° C. lower than the peak temperature of the maximum endothermic peak as determined by differential scanning calorimetric (DSC) measurement (namely, the melting point), followed by heating and holding at a temperature (T2) that is 10° C. to 2° C. lower than the peak temperature of the maximum endothermic peak as determined by DSC measurement of the heated toner composition. In this method, although heat-resistant storability and low-temperature fixability of the toner can be improved, in addition to requiring heating for a long period of time, the number of steps increases and the procedure is complex.
On the other hand, Japanese Translation of PCT Application No. 2009-504897 proposes a technology for crystallizing a molten polyethylene resin by holding in a high-pressure environment followed by cooling. According to this method, differing from the lamellar structure attributable to typical polymer chain folding, an extended chain crystalline structure is described as being adopted in which polymer chains are fully extended. This crystallization demonstrates a higher degree of crystallinity than ordinary crystallization at atmospheric pressure. However, the use of a crystallization technique requiring melting was difficult to apply to granules in the manner of toner in terms of maintaining the particle state of the toner.
As has been described above, production methods for adequately demonstrating the inherent performance of crystalline resin and sustaining stable low-temperature fixability and heat-resistant storability of toner over a long period of time continue to have problems.